Medication errors in ambulatory care settings represent an ongoing patient safety challenge. This study characterizes ambulatory care adverse drug events reported to a large patient safety organization between May 2012 and October 2018. Anticoagulants, antibiotics, hypoglycemics, and opioids were the most commonly involved medication classes. Contributing factors included prescribing errors, failure to review clinical contraindications or drug-drug interactions, and lack of patient education or communication.